1. Technical Field
The present invention relates to an intermetallic compound possessing superelastic properties, shape memory alloys formed therefrom, and the resulting shape memory articles.
2. Background of Related Art
Alloys which are capable of transforming between martensitic and austenitic parent phases are generally able to exhibit a shape memory effect. The transformation between phases may be caused by a change in temperature. For example, a shape memory alloy in the martensitic phase will begin to transform to the austenitic phase when its temperature rises above the austenite start temperature, As, and the transformation will be complete when the temperature rises above the austenite finish temperature, Af. The forward transformation will begin when the temperature drops below the martensite start temperature, Ms, and will be complete when the temperature drops below the martensite finish temperature, Mf. The temperatures Ms, Mf, As and Af define the thermal transformation hysteresis loop of a shape memory alloy.
Under certain conditions, shape memory alloys exhibit pseudoelasticity, which does not rely on temperature change in order to accomplish shape change. A pseudoelastic alloy is capable of being elastically deformed far beyond the elastic limits of conventional metals. Certain alloys, however, are capable of exhibiting pseudoelasticity of two types.
A first type of pseudoelasticity, xe2x80x9csuperelasticityxe2x80x9d (also sometimes referred to as non-linear pseudoelasticity), arises in appropriately treated alloys while they are in their austenitic parent phase at a temperature which is greater than As and less than Md (Md is the maximum temperature at which the transformation to the martensitic phase can be induced by the application of stress). Superelasticity can be achieved when the alloy is annealed at a temperature which is less than the temperature at which the alloy is fully recrystallized. Alternative methods of creating superelasticity in shape memory alloys are, for example, solution treating and ageing, alloying, etc. An article may be provided with a desired configuration by holding it in that configuration during annealing, or during solution treatment and ageing. An article formed from an alloy which exhibits superelasticity can be deformed substantially reversibly up to 11 percent or even more.
A second type of pseudoelasticity is xe2x80x9clinear pseudoelasticityxe2x80x9d. In contrast to superelasticity, xe2x80x9clinear pseudoelasticityxe2x80x9d is believed not to be accompanied by a phase change. It is exhibited by shape memory alloys which have been cold worked in the martensitic phase, but have not been annealed in the manner discussed above for superelastic behavior. An article formed from an alloy which exhibits linear pseudoelasticity can be deformed substantially reversibly by 4 percent or even more.
Examples of known metallic materials used to form shape memory alloys known in the prior art include nickel-titanium (Nitinol), nickel-titanium-vanadium. nickel-titanium-zirconium, nickel-titanium-zirconium-copper, indium-thallium, and gold-copper-zinc. These alloys have been employed in a wide variety of mechanical, electrical and medical/dental applications, e.g., connectors, couplings, window openers, valve switches, heat-actuated water sprinklers, safety switches, and orthodontal appliances. However, due to their high strength properties, these alloys cannot be deformed at low stress levels such as that encountered in, for example, gaskets or seals. In the case of gaskets, the deformation should be restricted to the gasket and not the flanges. Since these alloys possess high strength and/or high hardness, permanent deformation of the flanges will result. Therefore, these alloys would not be useful when employed as a gasket or seal.
It would therefore be desirable to have a shape memory alloy which can be deformed at low stress levels therefore making the alloys useful as gaskets or seals.
We have discovered that intermetallic compounds derived from Auxe2x80x94In2 exhibit superelasticity such that the intermetallic compounds possess shape memory characteristics when deformed at low stress levels yet also provide sufficient strength to maintain its shape when employed as a gasket or seal. In one embodiment, an Auxe2x80x94In2 intermetallic compound possessing shape memory characteristics is provided which comprises Au and In in a near atomic ratio of about 1:2 which has been heated to a predetermined temperature to melt and form a liquid alloy mixture and allowed to cool at room temperature.
In a second embodiment, a shape memory alloy is provided which includes at least a first phase having shape memory properties containing Au and In with additional alloying elements such as copper and/or nickel and a second inactive carrier phase containing elements selected from the group consisting of Au, In, Ni and Cu. These alloys are advantageously formed as a gasket or seal which possesses pseudoelastic, i.e., superelastic, properties. The seals are particularity useful for liquid oxygen systems.